This invention relates to a method of charge inverted mass spectroscopy that is characterized by its ability to differentiate between isomers and other substances that have been impossible to differentiate by the conventional mass spectroscopic techniques. The main thrust of mass spectroscopy as compared with other analytical techniques is that it is capable of determining the mass of the molecule of which a certain substance is made while allowing for microanalysis of the substance.
The conventional techniques of mass spectroscopy have the major drawback of permitting only low resolution in isomer differentiation. Having high resolution in isomer differentiation, the method of the invention allows for differentiation, identification and quantitative determination of many substances in small quantities and finds utility in those areas where analyses have been performed by chromatography and mass spectroscopy.
Dioxins have different forms of isomers depending upon the substitution position of chlorine atoms and the level of toxicity differs considerably from one isomer to another. With the present technology, it is an extremely painstaking job to separate dioxin isomers and determine their quantities. The efficacy and toxicity of medicines are also variable by great extent from one isomer to another. The present invention offers the advantage that individual isomers can be differentiated and quantitatively determined from much smaller quantities of samples than have been required in the prior art technology.
Recent improvements on mass spectroscopic technology have made it possible to determine the molecular weights of molecular species of different masses by measuring mass spectra using various ionization methods. Even molecules having the same mass number can be differentiated by electron impact spectra if they differ in elemental composition and other factors. However, difficulty is often encountered in differentiating and determining the structures of isomers having the same mass number but different structures.
In collision-induced dissociation (CID), generated ions are bombarded against a target such as a rare gas to produce ions of the same polarity by dissociation and as for certain molecules, their isomers can be differentiated by analyzing the spectra of the thus product ions. However, many substances still exist that defy differentiation of their isomers by CID. An improved modification of mass spectroscopic technology is desired that retains its capability for microanalysis and which yet allows for higher resolution in isomer differentiation.
CID is a technique relying upon the dissociation of generated ions. Having electrical charge, ions can be analyzed by electromagnetic means such as an electric or magnetic field and, in addition, they are readily detectable. These features make CID suitable for microanalysis. Radical ions, as contrasted with neutral species, have one or more electrons in excess or deficiency, so the activation barrier in the isomerization of radical ions is sufficiently lower than that for neutral species and the former are by far more likely to be isomerized than the latter. xe2x80x9cNeutral speciesxe2x80x9d is the generic term for electrically neutral particles and covers not only atoms, molecules, radicals and clusters but also other excited particles that are electrically neutral.
Another feature of CID is that ions are excited to varying internal energy levels and they are dissociated to give a spectrum comprising one dissociated ion superposed on another. This means that although microanalysis is possible by CID, the resolution in isomer differentiation is so low that certain compounds are even impossible to identify.
With a view to dissociating ions of neutral species in spite of high isomerization barrier, the present invention employs a single-electron transfer reaction between an incident positive ion and a target such as an alkali metal that has low ionization energy. In the invention, neutralization with the target takes place as a near resonant reaction whose probability of occurrence is high. In addition, the generated neutral species in the excited state has a narrow enough energy distribution to increase the likelihood for the occurrence of a specific dissociation.
The present invention is further characterized in that the neutral species of ion that has been generated by dissociation undergoes a second electron transfer reaction with the target to become a negative ion, which is analyzed and determined quantitatively by mass spectroscopy in an electric or magnetic field. This feature contributes to provide a higher resolution in isomer differentiation than the conventional mass spectroscopic technology. The method of the invention performs analysis and detection of a particular ion itself and, hence, it allows for analysis and quantitative determination in small quantities, which is one of the salient features of mass spectroscopy.
In the method of the invention, a positive ion is generated from an ion source that ionizes a substance such as an isomer, the generated positive ion is mass separated by mass spectroscopy in an electric or magnetic field, the mass separated positive ion is launched into a target chamber filled with a target in the form of alkali metal vapor, charge inversion is allowed to occur in the target chamber to produce a negative ion, the produced negative ion is taken out of the target chamber, and the mass spectrum of the recovered negative ion is analyzed to identify the substance of interest and determine its quantity.
When a substance is ionized, two types of ions are produced; one is ions that result from the intact substance (and which are commonly called xe2x80x9cparent ionsxe2x80x9d) and the other is various ions that result from the broken substance (xe2x80x9cfragment ionsxe2x80x9d). The characteristics of the substance are best retained by the parent ions, so in the present invention, the parent ions are selectively introduced into the target chamber. However, the fragment ions are also introduced into the target chamber since they represent the characteristics of a partial structure of the substance.
In electron impact ionization, the parent ions best reflect the structure of the substance. In other ionization techniques such as chemical ionization (CI) and fast atom bombardment (FAB), a hydrogen ion or an alkali metal ion are attached to the molecule to produce a quasi-molecular ion, which best reflects the structure of the substance (its molecule). Hence, in the present invention, the quasi-molecular ion can also be introduced into the target chamber.